1. Field of the Invention
The present invention relates to sources of optical pulses and methods of generating optical pulses, particularly tunable pulses, by use of the soliton-self-frequency shifting effect in an optical amplifier based on doped holey fiber.
2. Description of the Related Art
Wavelength tunable ultrashort (femtosecond and picosecond duration) optical pulse sources have applications in areas as diverse as ultrafast spectroscopy, materials processing, optoelectronics, nonlinear optics and optical chemistry. Traditionally, femtosecond (fs) pulse sources have been based on bulk crystal materials (most commonly Ti:sapphire), and have employed passive mode-locking techniques, such as Kerr-lens mode-locking, that make use of fast intracavity saturable absorber effects. Whilst excellent performance characteristics have been achieved, and successful commercial products and application areas have been developed, these traditional sources offer a limited range of directly accessible wavelengths and continuous broadband tuning ranges, particularly above 1.1 xcexcm. In general, extending this femtosecond technology to obtain broader tuning ranges and longer wavelengths requires the use of bulk parametric nonlinear devices such as optical parametric oscillators (OPOs), generators (OPGs), or amplifiers (OPAs), pumped by a bulk femtosecond laser. Such devices add to the complexity and cost, and increase the physical size of the overall system. Moreover, bulk crystal lasers require high-precision alignment and are often pumped by expensive, high-maintenance gas lasers. An alternative approach to obtain broadband tunability is to first generate a broadband supercontinuum spectrum and to spectrally filter out (pulsed) radiation at the desired wavelength (or wavelengths). This technique is commonly referred to as spectral slicing. A supercontinuum spectrum itself (unfiltered) also has many applications including metrology and optical coherence tomography and convenient/practical means to generate such broadband spectra are required. For many applications the spatial mode quality of the supercontinuum beam is an important issue and generally a high quality mode (e.g. a single transverse mode) across all wavelengths is required.
An attractive way of achieving tunability from an ultrashort pulse system is to use the soliton-self-frequency shift (SSFS) effect. The discovery of the SSFS effect in optical fibers was first reported in 1985-1986, and opened up the exciting possibility of obtaining widely wavelength tuneable femtosecond soliton pulses from a variety of optical sources, including fiber-based sources. Femtosecond pulses launched in a suitable optical fiber will propagate as solitons, and Raman frequency shifting within the spectra of the individual solitons gradually alters the wavelength of the pulses. The amount of alteration, or tuning, is governed by factors including pulse power, fiber material and fiber length.
Optical fiber requires certain characteristics to support the SSFS effect. A sufficient level of optical nonlinearity is required to enable solitons to develop and propagate. The nonlinearity experienced by a pulse depends on the amount of energy in the pulse, so pulses propagating in the fiber therefore need to have sufficient energy for soliton formation. Also, to obtain the self-frequency shift, the fiber needs to have anomalous dispersion over the wavelength range of interest, namely the wavelength of the initial launched pulses, and the required tuning range.
Various practical demonstrations of the SSFS effect have been reported. Nishizawa and Goto [1, 2] have reported SSFS in a standard polarization maintaining fiber, using femtosecond pulses from an erbium-doped fiber laser. A soliton output tunable over 1.56 to 1.78 xcexcm was achieved. A further device using nJ pulses from an erbium-doped fiber laser and SSFS in a standard silica fiber has been reported by Fermann et al [3]. An alternative arrangement by Liu et al [4] uses a tapered microstructured silica fiber to provide SSFS of femtosecond nJ pulses from a Ti:sapphire-pumped OPO. The tapering and microstructuring of the fiber is used to give a large anomalous dispersion with a flattened profile in the wavelength region of interest, 1.3 to 1.65 xcexcm. In each case, tuning is provided by varying the power of the pulses launched in the fiber.
However, the use of conventional silica fiber for SSFS has limitations, in that it is only possible to obtain anomalous dispersion for wavelengths beyond xcx9c1.3 xcexcm. This precludes the use of SSFS for achieving tunability in the desirable but difficult to access wavelength region of 1 to 1.3 xcexcm. Also, the anomalous dispersion and nonlinearity available may be less than optimal for any particular tuning range of interest.
Furthermore, as mentioned, relatively high pulse energies are required to exploit the fiber nonlinearity sufficiently to achieve soliton propagation. Typically, nJ pulses have been used in the prior art. This requirement puts limitations on the laser sources which can be used to drive the SSFS effect. Also, this may have an adverse effect on the available tuning, because tuning is typically achieved by varying the pulse energy/power.
The present invention seeks to address the limitations of the prior art by providing a more versatile tunable ultrashort optical pulse source based on the SSFS effect. This is achieved by using, as the SSFS medium, in one embodiment, a holey fiber having a doped core and configured as an amplifier. The unusual properties of holey fibers, and also, to some extent, tapered fibers, can be exploited to provide a fiber with anomalous dispersion at virtually any desired wavelength, so that it is possible to access wavelength regions not attainable with conventional fibers, including 1 to 1.3 xcexcm and below. Also, such fibers can be tailored to have a much greater nonlinearity than conventional fibers, so that lower pulse energies can be used to achieve solitonic operation. The present invention can be readily utilised using pJ pulse energies.
The ability to use lower pulse energies is further enhanced by configuring the fiber as an amplifier. Providing internal amplification in this way allows the use of low energy pulses which are then amplified within the fiber until they have sufficient energy to experience the nonlinearity of the fiber, propagate as solitons, and then undergo SSFS. Thus, a wide range of pulsed sources are suitable for use in the present invention, given the minimal limitations on both power and wavelength. For example, wavelength shifting of relatively low energy pulses directly from a simple diode-pumped fiber oscillator is possible. Moreover, the use of an amplifier allows the tuning of the SSFS to be achieved by varying the power of the amplifier pump source, instead of the prior art method of varying the output power of the pulse source. This decoupling of the wavelength tuning from the operation of the pulse source can be advantageous in practice since the fundamental pulse source is left running and does not need to be adjusted at all to effect wavelength tuning of the system output Furthermore, the distributed amplification process offers tuning over a broader frequency range than has hitherto been possible with the passive SSFS devices of the prior art; an embodiment of the present invention has produced femtosecond pulses at wavelengths as long as 1.58 xcexcm from pulses having a wavelength of 1.06 xcexcm, corresponding to a frequency shift of 69 THz, which is one third the frequency of the input pulses. The source configuration can also be used to generate pulsed output with an ultra-broadband optical spectrum. The generation of this ultra-broad optical spectrum relies upon supercontinuum generation/effects within these highly nonlinear fibers.
Accordingly, a first aspect of the present invention is directed to a source of optical pulses, comprising: an optical source operable to generate ultrashort optical pulses at a first wavelength; and an optical fiber amplifier comprising an optical fiber and a pump source operable to deliver pump radiation to the optical fiber, the optical fiber having a core containing a dopant to provide optical gain at the first wavelength, and anomalous dispersion over a wavelength range including the first wavelength and a second wavelength, and being arranged to receive the ultrashort optical pulses, amplify the ultrashort optical pulses, and alter the wavelength of the ultrashort optical pulses to at least the second wavelength by the soliton-self-frequency shifting effect.
The optical fiber may be configured as a microstructured fiber containing an array of air holes running along the length of the fiber, or alternatively as a tapered fiber. In either case, it is possible to tailor the dispersion properties of the fiber to permit use of the SSFS effect over a desired wavelength range, so that pulses having a particular wavelength can be generated.
In one embodiment, the dopant comprises ytterbium ions, to provide optical gain at a wavelength of approximately 1 xcexcm. Wavelengths around one micron cannot be accessed using SSFS in passive devices using standard optical fiber owing to the dispersion properties of such fiber. Hence, ytterbium doping gives gain at these wavelengths, which together with suitable fiber dispersion characteristics, can be used to generate these wavelengths via SSFS.
Alternatively, the dopant comprises ions of one or more of erbium, neodymium, ytterbium, holmium, thulium, praseodymium, germanium, aluminium, boron, samarium, lead and tin. These other dopants allow/enhance gain, and subsequent wavelength shifting, at a variety of wavelengths. For example, aluminium can enhance/extend the gain bandwidth provided by rare earth dopants. Also, some of the dopants can be used to improve the nonlinear coefficient, and/or enhance the Raman gain in addition to or instead of enhancing the optical gain.
Advantageously, the optical source comprises a laser having an optical gain medium in the form of an optical fiber. With appropriate selection of the pump laser, this gives a wholly fiber-based device, with the known advantages of such systems, such as stability and robustness.
Further, the laser may have an optical gain medium comprising an optical fiber doped with ions of ytterbium and operable to generate ultrashort optical pulses at a wavelength of approximately 1 xcexcm. With appropriate choice of fiber dopant, such as ytterbium, to give gain at this wavelength, the source can produce pulses in the 1.1 to 1.3 xcexcm range, which is difficult to access using prior art devices.
In a preferred embodiment, the pump radiation delivered to the optical fiber can be varied in power, so as to vary the second wavelength within the wavelength range of the anomalous dispersion of the optical fiber of the optical fiber amplifier. This gives an effective way of providing a tunable output, without any disturbance to the optical source being necessary.
In one embodiment, the ultrashort optical pulses are delivered to the optical fiber of the optical fiber amplifier with sufficient power for the soliton-self-frequency shifting effect to alter the wavelength of the ultrashort pulses to the second wavelength and to one or more additional wavelengths within the wavelength range of the anomalous dispersion of the optical fiber of the optical fiber amplifier. Operation in this regime gives an output of multicolored solitons.
In a further embodiment, the ultrashort optical pulses are delivered to the optical fiber of the optical fiber amplifier with sufficient power for the soliton-self-frequency shifting effect to alter the wavelength of the ultrashort pulses to a broadband continuous spectrum. Supercontinuum spectra of this type have many applications, including metrology, spectroscopy and optical coherence tomography.
The source of optical pulses may further comprise an optical pre-amplifier operable to receive the ultrashort optical pulses from the optical source and amplify the ultrashort optical pulses before the ultrashort optical pulses are received by the optical fiber of the optical fiber amplifier. Pre-amplification can be used to increase the energy of the ultrashort optical pulses if necessary, for example, for operation in the broadband regime. The pre-amplifier may comprise an optical fiber amplifier or a bulk optic amplifier.
A second aspect of the present invention is directed to a method of producing optical pulses, comprising: generating ultrashort optical pulses at a first wavelength; amplifying the ultrashort optical pulses in an optical fiber amplifier; and altering the wavelength of the ultrashort optical pulses from the first wavelength to at least a second wavelength by the soliton-self-frequency shifting effect within the optical fiber amplifier; the optical fiber amplifier comprising an optical fiber having a core containing a dopant to provide optical gain at the first wavelength, and anomalous dispersion over a wavelength range including the first wavelength and a second wavelength.
The method may further comprise varying an amount of pump radiation delivered to the optical fiber amplifier so as to vary the second wavelength. This is a simple way of achieving a tunable output, without the need to impinge on the generation of the ultrashort optical pulses.
The desired dispersion characteristics may be attained by configuring the optical fiber of the optical fiber amplifier as a microstructured fiber containing an array of air holes running along the length of the fiber, or alternatively. as a tapered fiber.
The ultrashort optical pulses may be generated from a laser having an optical gain medium in the form of an optical fiber, and the optical fiber may be doped with ions of ytterbium.
In one embodiment, the method further comprises delivering the ultrashort optical pulses to the optical fiber amplifier with sufficient power for the soliton-self-frequency shifting effect to alter the wavelength of the ultrashort pulses to the second wavelength and to one or more additional wavelengths within the wavelength range of the anomalous dispersion of the optical fiber of the optical fiber amplifier.
In an alternative embodiment, the method further comprises delivering the ultrashort optical pulses to the optical fiber amplifier with sufficient power for the soliton-self-frequency shifting effect to alter the wavelength of the ultrashort pulses to a broadband continuous spectrum.
The method may further comprise amplifying the ultrashort optical pulses in an optical pre-amplifier before amplifying the ultrashort optical pulses and altering the wavelength of the ultrashort optical pulses in the optical fiber amplifier. The pre-amplifier may comprise, for example, an optical fiber amplifier, or a bulk optic amplifier.